This invention generally relates to sootblowers which are used to project a stream of a sootblowing medium against internal surfaces of a combustion device for cleaning the surfaces. In particular, this invention concerns a nozzle assembly which produces a cleaning effect which is superior to nozzles of conventional design.
Sootblowers are used to project a stream of cleaning medium such as water, air or steam against heat transfer surfaces within a combustion device such as large scale boilers to cause slag and ash encrustations to be removed. The cleaning medium impacting against the surfaces causes adhering layers to be removed. Various types of sootblowers are used. One general category of sootblowers is known as the long retracting type. These devices have a retractable lance tube which is periodically advanced into and withdrawn from the boiler, and may be simultaneously rotated such that one or more nozzles on the lance tube project a jet of cleaning medium which traces a helical path. In typical retracting sootblowers, a feed tube is held stationary relative to the sootblower frame. One end of the feed tube is supplied with the cleaning medium through a poppet valve. The lance tube slidably over-fits the feed tube and its longitudinal sliding and rotational motion is controlled by a carriage which moves along tracks on the sootblower frame. The cleaning medium supplied to the feed tube in turn pressurizes the hollow inside of the lance tube. The cleaning medium escapes from the lance tube through one or more nozzles which direct the spray against the surfaces to be cleaned. At the conclusion of a cleaning cycle, the lance tube is retracted and withdrawn from the combustion device to avoid exposure to intense heat which would destroy the lance tube.
Cleaning of slag and ash encrustations within the internal surfaces of a combustion device occurs through a combination of mechanical and thermal shock caused by the impact of the cleaning medium. In order to maximize this effect, designers of sootblowing equipment strive to design lance tubes and nozzles which produce a coherent stream of cleaning medium having a high peak impact pressure (i.e. maximum dynamic pressure at point of contact) and which can clean surfaces a long distance from the nozzle.
Various cleaning mediums are used in sootblowers. Steam and air are used in many applications. In order to maximize the cleaning effect, a stream which is fully expanded as it exits the nozzle is desired. Full expansion refers to a condition in which the static pressure of the stream exiting the nozzle approaches that of the ambient pressure surrounding the lance tube. Classical nozzle design theory for compressible fluids such as air or steam require that the nozzle have a throat with an expanding cross-sectional area which allows the pressure of the fluid to be reduced as it passes through the nozzle. The rate of expansion of the nozzle throat cross-sectional area is however limited by a desire to minimize boundary layer separation of the stream flowing through the nozzle, which limits the divergence angle of the nozzle throat surfaces. The occurrence of boundary layer separation leads to a turbulent flow regime which adversely affects the stream cleaning capabilities. Unfortunately, such conventional full expansion nozzles cannot readily be incorporated into many sootblower lance tubes since they tend to be longer than can be incorporated in the lance tube. Such constraints result since it is necessary to move the lance tube into and out of the combustion device through a small access port which limits the extent to which a nozzle can extend beyond the outside diameter of the lance tube. The length of the nozzle is also limited by a desire to ensure that the inlet end of the nozzle within the lance tube does not extend so far across the inside diameter of the lance tube that flow area through the lance tube is restricted. Accordingly, conventional full expansion nozzles cannot generally be incorporated into most retracting sootblowers.
By providing a sootblower nozzle having higher peak impact pressure and penetration ability, enhanced cleaning performance results which may permit a lower consumption of cleaning medium which can translate into a higher overall efficiency of the associated boiler. Moreover, by providing a more penetrating stream, it may be possible to decrease the number of sootblowers in a given area of a boiler required to provide a desired cleaning effect thereby providing considerable savings to the boiler operator in terms of capital investment and operating costs.
In accordance with the present invention, improvements over existing sootblower nozzles are provided. The nozzles according to this invention emulate the characteristics of a conventional full expansion nozzle while having a short length or "low profile" which can be incorporated into a sootblower lance. The nozzles of this invention have a nozzle throat with a centrally located plug which produces an annular flow passageway. By providing a diverging surface on the inside of the hollow nozzle shell, and a converging surface on the plug, the rate of expansion of cross-sectional area can be increased without violating divergence angle limitations. Such low profile nozzles have been evaluated in connection with this invention and found to provide performance which approaches that of conventional full expansion nozzles which have a completely open throat area.
This invention further encompasses various approaches toward mounting the plug within the nozzle throat. In one embodiment, the plug is supported by the back wall of the lance tube, whereas in another embodiment, the plug is supported by a radially extending supporting vane. In an additional embodiment, a one piece double-ended plug is used for diametrically opposed nozzles. While nozzles with annular flow passages are know generally, for example, modern jet engines can be thought of as such a nozzle, they have not been adapted to the environment of a sootblower lance tube where they provide unique benefits.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.